376 ROBERT W. HEGNER 



II. The origin and significance of the Keimbahn-determinants in animals. . . . 432 



1. The Keimbahn in the Crustacea 433 



2. The Keimbahn in the Nematoda 442 



3. The Keimbahn in Sagitta 446 



4. The Keimbahn in vertebrates 449 



5. The Keimbahn in other animals 456 



6. The genesis, localization, distribution, and fate of the Keimbahn-;le- 



terminants 460 



A. The genesis of the Keimbahn-determinants 460 



' a. From nuclear substances 462 



b. From cytoplasmic or extracellular nutritive substances .... 469 



c. From a differentiated part of the cytoplasm 474 



B. The localization of the Keimbahn-determinants 478 



C. The fate of the Keimbahn-determinants 482 



7. Conclusions and summary 485 



Literature cited r 489 



GENERAL INTRODUCTION 



Studies of the history of the germ cells in animals have proven 

 that in many cases these cells originate in a perfectly definite 

 way and at such an early embryonic period as to represent the 

 first cellular differentiation that ta|ves place in ontogeny. In 

 certain animals such a determinate segregation of the germ cells 

 cannot be established with the data available without certain 

 assumptions to which objections may be made. Limiting 

 ourselves, therefore, to the instances where the germ cell cycle 

 is completely known, it is possible to divide the history of the 

 germ cells from one generation to the next into the following 

 periods : 



1. Primary cellular differentiation, i.e., the formation of one 

 or more primordial germ cells during the segmentation of the egg; 



2. A short period during which in some cases the primordial 

 germ cells increase slightly in numbers by mitosis; 



3. A long period of rest characterized by cessation of cell 

 division, either active or passive change of position, separation 

 of the germ cells into two groups which become the definitive 

 germ glands, accompanied by the general growth of the embryo 

 until the larval stage is almost attained; 



4. Multiplication by mitosis of the primitive oogonia or 

 spermatogonia to form a definite number (Miastor and perhaps 



